Important Interactions
Reviewed supplement and medication pairs classified as avoid, use with caution, or monitor -- each linking to its full sourced interaction page.
These classifications come directly from the sourced evidence behind each pair -- they are not a ranking of every possible combination. A pair that is not listed here has either not yet been reviewed, or was reviewed and no documented interaction was identified in the sources checked. Neither of those is the same as "safe" -- see methodology and the medical disclaimer. For sourced timing/spacing considerations specifically, see Timing & Absorption.
Avoid combining
Avoid combining
Sources document a clinically significant reason these should generally not be combined without a clinician's guidance.
Use caution
Use caution
Sources document a real interaction that warrants care -- dose adjustment, spacing, or professional guidance before combining.
Atorvastatin + Warfarin
Use cautionAtorvastatin is listed as a CYP3A4 inhibitor on warfarin's FDA label, a class that can increase warfarin's effect.
Caffeine + Warfarin
Use cautionCaffeine is listed as a CYP1A2 inhibitor on warfarin's FDA label, a class that can increase warfarin's effect.
Calcium + Levothyroxine
Use cautionCalcium can reduce how much levothyroxine your body absorbs -- the FDA label recommends spacing them at least 4 hours apart.
Ciprofloxacin + Calcium
Use cautionCalcium can reduce how much ciprofloxacin your body absorbs -- FDA labeling recommends spacing them several hours apart.
Doxycycline + Calcium
Use cautionCalcium can reduce how much doxycycline your body absorbs -- FDA labeling documents this directly.
Ciprofloxacin + Zinc
Use cautionZinc can reduce how much ciprofloxacin your body absorbs -- NIH guidance recommends spacing them several hours apart.
Ciprofloxacin + Warfarin
Use cautionCiprofloxacin can enhance warfarin's blood-thinning effect -- named directly on ciprofloxacin's FDA label.
Ciprofloxacin + Iron
Use cautionIron can reduce how much ciprofloxacin your body absorbs -- FDA labeling recommends spacing them several hours apart.
Ciprofloxacin + Magnesium
Use cautionMagnesium can reduce how much ciprofloxacin your body absorbs -- FDA labeling recommends spacing them several hours apart.
CoQ10 (Coenzyme Q10) + Warfarin
Use cautionCoQ10 may decrease warfarin's blood-thinning effect -- named directly on warfarin's FDA label.
Doxycycline + Iron
Use cautionIron can reduce how much doxycycline your body absorbs -- FDA labeling documents this directly.
Doxycycline + Magnesium
Use cautionMagnesium can reduce how much doxycycline your body absorbs -- FDA labeling documents this directly.
Doxycycline + Zinc
Use cautionZinc may reduce doxycycline absorption -- zinc's own NIH fact sheet discusses tetracycline-class antibiotics as a group.
Levothyroxine + Iron
Use cautionIron can reduce how much levothyroxine your body absorbs -- FDA labeling recommends spacing them at least 4 hours apart.
Lisinopril + Spironolactone
Use cautionCombining lisinopril and spironolactone can raise potassium to dangerous levels -- named directly on both drugs' FDA labels.
Omeprazole + Warfarin
Use cautionOmeprazole can increase warfarin's effect and bleeding risk -- named directly on omeprazole's FDA label with a monitoring recommendation.
Sertraline + St. John's Wort
Use cautionCombining sertraline with St. John's Wort raises the risk of serotonin syndrome -- named directly on sertraline's FDA label.
Sertraline + Warfarin
Use cautionSertraline is named directly on warfarin's FDA label as a drug that can increase bleeding risk.
Warfarin + St. John's Wort
Use cautionSt. John's Wort can weaken warfarin's blood-thinning effect -- this is named directly on warfarin's FDA label.
Vitamin K + Warfarin
Use cautionVitamin K intake needs to stay consistent while taking warfarin -- it does not need to be avoided.
Monitor
Monitor
Sources document an interaction worth watching for -- monitoring a lab value, symptom, or effect while combining.
Atorvastatin + CoQ10 (Coenzyme Q10)
MonitorAtorvastatin has been shown to lower blood CoQ10 levels in a clinical study.
Calcium + Caffeine
MonitorCaffeine may interfere with how much calcium your body absorbs.
Ciprofloxacin + Caffeine
MonitorCiprofloxacin reduces how quickly your body clears caffeine -- named directly on ciprofloxacin's FDA label.
Calcium + Iron
MonitorCalcium may reduce how much iron your body absorbs from a supplement or meal.
Calcium + Omeprazole
MonitorLong-term, high-dose omeprazole use is associated with an increased risk of bone fracture -- its FDA label documents this directly.
Copper + Zinc
MonitorTaking high doses of zinc supplements for a long period can reduce copper absorption and lead to copper deficiency.
Omeprazole + Iron
MonitorOmeprazole can reduce iron absorption -- named directly on its FDA label as a drug dependent on gastric pH.
Zinc + Iron
MonitorHigh doses of iron can reduce how much zinc your body absorbs.
Sertraline + Levothyroxine
MonitorStarting sertraline can increase how much levothyroxine you need -- named directly on levothyroxine's FDA label.
Omeprazole + Levothyroxine
MonitorOmeprazole, as a proton pump inhibitor, may reduce levothyroxine absorption -- named on levothyroxine's own FDA label at the drug-class level.
Magnesium + Levothyroxine
MonitorMagnesium-hydroxide antacid products (not standalone magnesium supplements) can reduce levothyroxine absorption.
Lisinopril + Potassium
MonitorLisinopril can raise potassium levels -- its FDA label recommends monitoring, particularly if combined with a potassium supplement.
Magnesium + Zinc
MonitorVery high doses of supplemental zinc can reduce the body's absorption of magnesium.
Magnesium + Omeprazole
MonitorOmeprazole use can lower magnesium levels -- its FDA label documents this directly and recommends monitoring in some patients.
Vitamin B12 + Metformin
MonitorLong-term metformin use can lower vitamin B12 levels -- its FDA label recommends periodic monitoring, not avoidance.
Vitamin B12 + Omeprazole
MonitorLong-term omeprazole use can reduce vitamin B12 absorption -- its FDA label documents this directly.
Omeprazole + Potassium
MonitorOmeprazole's label advises telling your doctor if you have low potassium levels before starting treatment.
Not sure where your combination falls? Check it directly, or browse Supplement Safety and Medication Safety for broader guidance.